I am trying to build Python 3.6.5 from source, with Tcl 8.6.8 and Tk 8.6.8, and I get the same issue (Building on Ubuntu 18.04, tried with GCC 7.3 and 8.1 with same results). 
Run 'readelf -d` on 'build/lib.linux-x86_64-3.6/_tkinter.cpython-36m-x86_64-linux-gnu_failed.so' and I get three NEEDED entries:


 0x0000000000000001 (NEEDED)             Shared library: [libpython3.6m.so.1.0]
 0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]


Tcl and Tk libraries are not listed as needed shared libraries, which explains why settings runpaths (which I tried by setting CFLAGS) does not seem to work.
I'm comparing this to the ELF entries in Ubuntu's Python 3.6.5 tkinter module '/usr/lib/python3.6/lib-dynload/_tkinter.cpython-36m-x86_64-linux-gnu.so', which does have NEEDED entries for tcl8.6.so and tk8.6.so.